This invention relates to a binder for the granulation of a dry, stable particulate fertilizer, such as ammonium sulfate, utilizing conventional commercially available granulating devices, to prepare a relatively uniform blend of granules of a specific desired sizing for use as a fertilizing material.
A commonly utilized fertilizer material is dry ammonium sulfate, which for convenience is usually granulated to a specified size range for application. The grain size of ammonium sulfate varies considerably according to the method of manufacture. Usually, the crystal size can vary between 0.1 mm and 2 mm. The use of such small grain sizes in fertilizers presents a problem of blending with other granulated primary nutrients, with respect to segregation. Also, such small crystal sizes are not ideal for direct application to the field through fertilizer applicators.
The fertility of varying soils often requires the application of more than one nutrient for the correction of nutritional deficiencies in the soil. While it is common practice to produce granular, multiple-nutrient fertilizers, in which each granule contains the primary nutrients nitrogen, phosphorous, and/or potassium in a definite predetermined ratio, a specific ratio will not always meet the requirements of a specific soil. Thus, a producer of fertilizers has either to produce a great number of nitrogen-phosphorous-potassium type fertilizers in granular form, or to physically mix together the primary nutrients as individual granules. Granular ammonium sulfate would, if suitably available, be an appropriate primary nutrient source of nitrogen and sulfur for such blending.
These mechanically mixed fertilizers, or bulk blended fertilizers, allow the production of an infinite number of ratios in regard to their content of primary nutrients. Bulk blending is, however, only a practical proposition as long as the fertilizer materials used in the preparation of the bulk blended fertilizer are well granulated and have not only a close, but also a very similar, spectrum of sizings. Accordingly, it has become increasingly important to closely control the granule size of various nutrient materials. It has previously been difficult to achieve the formation of granules of ammonium sulfate within very narrow size ranges in a practical manner in conventional granulating devices, such as a rotating drum or pan, or a blunger. Attempts to granulate such salts in these devices have produced a wet crystalline pulp during the wetting of the cascading bed, which could not be coalesced and agglomerated into spheres. One process, which was developed by the National Fertilizer Development Center of the Tennessee Valley Authority (TVA), may be used for the production of a granular ammonium sulfate. In this process, sulfuric acid and ammonia are pre-neutralized in a reactor to an NH.sub.3 :H.sub.2 SO.sub.4 molar ratio of 1:1. The pre-neutralized fluid which usually contains less than 10 percent free water, and has a temperature of about 150.degree. C., is then fully ammoniated and granulated in a TVA drum granulator operating at a recycle ratio of about 1:1. The heat evolved during the ammoniation makes the drying of the granules superfluous. The granules discharged from the drum are cooled and screened. The over-size is disintegrated and recycled to the double-deck screen, while the fines are returned to the drum granulator. This process produces spherical granules, but its use is dependent upon the availability of cheap ammonia. In addition, the ammonium sulfate produced in this manner is usually not competitive with by-product ammonium sulfate from coke ovens.
In U.S. Pat. No. 4,277,253, a method is taught for the preparation of dry, stable granular material from water-soluble potassium and ammonium salts, or a mixture thereof, by subjecting a dispersion of crystals of the salt in an aqueous liquid to the action of a granulating device in the presence of calcium sulfate hemi-hydrate and a hydrophilic surface-active agent to effect agglomeration of said crystals, and thereafter subjecting the agglomerated crystals to a drying operation. Thus, gypsum and a hydrophilic surface-active agent are used in a dispersion of a salt, such as ammonium sulfate which is sprayed onto dry crystals in a drum or pan granulator or pug mill. This technique necessitates the addition of extra equipment to mill ammonium sulfate to a finer particle size, then dissolve the same to make a granulation fluid to spray onto other ammonium sulfate crystals. Also, high levels of calcium sulfate are required to give agglomerates of good quality, at the expense of greatly reduced nitrogen value.
What has been needed for some time has been a granulation technique which utilizes commonly operated granulating equipment, and is capable of recovering fines or reduced size ammonium sulfate, and agglomerating the same into a carefully controlled size range. Most fertilizer granulators operating in the United States are of the continuous ammoniation-granulation type. In such a plant, potassium sulfate and other dry nutrient-containing materials are measured continuously by gravimetric belt feeders. Liquid raw materials, such as anhydrous ammonia, nitrogen solutions, sulfuric acid, or phosphoric acid, are measured continuously by metering. Both liquid or dry materials are fed into a rotating drum or granulator, where products are intimately mixed and agglomeration takes place. After the initial granulation phase, the product passes through a rotary dryer and cooler. The cooled product is screened to separate the on-size product from over-size and fines. Over-size material is crushed and rescreened, and fines are recycled back into the ammoniator-granulator. The TVA and the United States Department of Agriculture have contributed a wealth of additional information to the granular process over the last three decades. Circular Z-18, published in November 1970 by the TVA and the National Fertilizer Development Center, entitled "Producing Granular Fertilizers," describes in detail typical granulation techniques for fertilizers.
Raw materials used in such granulation processes usually vary in size from as small as, for example, -325 mesh to as large as the minimum size specification of the finished product. Great care must be exercised in determining the proportions of various sized raw materials to be included in granulating product so as to produce a final mix which granulates at an efficient rate. Maximum level of on-size finished product, particularly when no binder is used, is produced when a variety of sizes of raw material is included in the raw material feed. This, however, tends to significantly increase the cost of the final product. At the same time, the specific nutrient material levels must be maintained at the required specification in order to meet the desired formulation.
Many raw material products available to the fertilizer granulator are more economically priced if the granulator can use reduced size or "fines" of the raw material. Raw materials such as ammonium sulfate, and others, are frequently available to the fertilizer granulator in a reduced sizing at a reduced cost. Maximum economy in such cases depends upon maximum use levels. In addition, there is often raw material available which is considered "distressed goods," as it is no longer in condition to be used for bulk blended product.
If a proper balance of fertilizer chemistry, binder, and raw material size balance is not met, granules which are formed during granulation are often weak and readily disintegrate to powder during handling and use. Dustiness in the product can become excessive, causing extreme handling and flow problems during transfer and in fertilizer application equipment. Certain other raw materials and raw material combinations, when used at the most economically desirable levels, draw excessive moisture to the finished granules, thus limiting storage and shelf life and causing excessive caking in storage bins, finished product piles, and fertilizer application equipment. One class of such ingredient which may be used in combination with ammonium sulfate, and which frequently exhibits this characteristic, is urea and urea combinations. A granular urea-containing composition encompassing calcium sulfate and calcium phosphate is taught in U.S. Pat. No. 4,283,423. Various possibilities have been proposed to avoid such caking when utilizing urea in fertilizer manufacture. They include "dusting" a conditioning agent on the manufactured granule. Such dusting agents include starch, clays, and calcium sources, as described in U.S. Pat. No. 3,332,827. This method is less than satisfactory, as it requires additional equipment and, at best, only coats particles of the granule, thus providing only partial relief from the caking.